We're in London and are on mains water and sewerage charges based on the rateable value of our terraced house. We do have a Victorian soakaway under the garden which takes rainwater run-off from the rear slope of our roof. Strictly speaking, I think we could claim a rebate on our sewage charges because we don't dump that rainwater into the sewers, but I've not tried that.
Ten years ago there was a lot of propaganda about the benefit of water meters. I plumbed in a water meter where our mains supply enters the house and wired it into a small data logger, leaving it running for a couple of months. The only people living here were the two of us and a daughter...….no swimming pools or koi ponds. The washing machine runs twice a week on average, and we have a bath or shower daily. If we were on a water meter, the data showed that we would be paying hundreds of pounds more each year compared to being on a fixed charge. I was surprised.